This project focuses on the systematic factors that affect linkage disequilibrium (Id) in different populations, different regions of the genome, and in regions of putative selection. The populations (about 40, with an average of more than 50 samples in each) represent a global sampling of our species. The 12 regions of the genome to be studied come from ten different chromosomes and each has a 200 kb span which may fully contain one or more haplotype blocks; and four of the 200 kb regions will be centered around genes with strong evidence of selection (around MC1R, LCT, CCR5, and HBB). All genomic regions to be studied will be typed for polymorphisms at 5-10 kb intervals in every population. We shall thus be able to (1) identify whether Id is distributed in blocks in the studied regions in all, most, or any populations, (2) determine the molecular extent of the blocks we find, (3) determine how generalizable across populations the block boundaries are, (4) examine any other pattern of Id decay if we do not find it to be punctate, and (5) examine the Id in and around loci with selection to identify "footprints" of selection.